As Dynamic Random Access Memory (DRAM) devices have become more highly integrated, device isolation methods have changed from the conventional LOCOS (local oxidation of silicon) technique to the STI (shallow trench isolation) technique. In case of the STI techniques, however, expansion of the oxide layer in the trench inner walls may cause a shallow pit in an active region. As a result, leakage currents may increase in the active region of a semiconductor substrate.
To reduce the above-mentioned problem, a nitride liner has been used as an oxidation masking layer to reduce subsequent oxidation of the trench inner wall. With the use of the nitride liner, the shallow pit can be reduced. The nitride liner, however, may be etched to a point below a surface of the semiconductor substrate (i.e., the nitride liner may be dented) during etching of an active nitride masking layer when using phosphoric acid. Thus, the dented nitride liner may cause a stringer residue in the subsequent step of etching a gate poly.
To reduce the described problems, a shallow isolation trench with a nitride liner having a thickness of less than 50 .ANG. has been proposed. See, for example, U.S. Pat. No. 5,447,884, the disclosure of which is hereby incorporated herein in its entirety by reference.
The method mentioned above includes the steps of: depositing a protective (masking) layer containing at least one layer of nitride on the semiconductor substrate; etching through the protective layer to form a set of isolation mask apertures; etching through the set of isolation mask apertures to form a set of isolation trenches; depositing a conformal liner of nitride having a thickness less than 50 .ANG.; depositing a CVD layer of oxide having a thickness sufficient to fill the set of isolation trenches; removing the portion of the CVD layer of oxide outside the set of isolation trenches to expose the at least one layer of nitride; and stripping the at least one layer of nitride in phosphoric acid. According to above method, the denting of the conformal nitride liner may be reduced by using a very thin nitride liner as an oxidation masking layer. The conformal nitride liner, however, may be so thin that it is insufficient to prevent oxidation of the trench inner wall.
Therefore, a method is needed which not only reduces denting the nitride liner but also reduces oxidation of the interior walls of the trench.